r7596: next step in ldap cleanup. I'm aiming to get rid of the cut&pasted

r7596: next step in ldap cleanup. I'm aiming to get rid of the cut&pasted
ldif parsing code in libcli/ldap/ldap_ldif.c, and instead use the ldb
ldif code. To do that I have changed the ldap code to use 'struct
ldb_message_element' instead of 'struct ldap_attribute'. They are
essentially the same structure anyway, so by making them really the
same it will be much easier to use the ldb code in libcli/ldap/

I have also made 'struct ldb_val' the same as a DATA_BLOB, which will
simplify data handling in quite a few places (I haven't yet removed
all the code that maps between these two, that will come later)